1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically to an image forming apparatus for forming on a photosensitive member a monochromatic image or a color image represented by a plurality of color components.
2. Description of Related Art
Among copying machines and multi function printers (MFPs) having a copying function using a conventional image forming apparatus, a type capable of measuring a toner consumption is known (for example, see Japanese Patent Application Laid-Open No. 4-261566 (1992)). In such a type of multi function printer, by measuring the toner consumption, it becomes possible to charge in accordance with the toner consumption and perform maintenance based on the toner consumption. The toner consumption can be calculated, for example, based on the cumulative value by totaling pixel values (density values) of pixels of an image formed on a photosensitive member.
Recently there have become popular copying machines or multi function printers utilizing an image forming apparatus capable of performing color printing. When performing color printing utilizing an image forming apparatus, for example, laser beam is irradiated from laser diodes corresponding to respective C (Cyan), M (Magenta), Y (Yellow), K (Black) onto a photosensitive member corresponding to respective C, M, Y, K, whereby electrostatic latent images of color components are formed, and images obtained by developing the electrostatic latent images with toners of colors corresponding thereto are once formed on the photosensitive member. Thereafter, images for each color are registered and transferred from the photosensitive member onto a sheet. There has been also known an image forming apparatus in which, for example, two laser diodes for black are provided, so that the separate diodes form an odd number line (K1) and an even number line (K2), respectively, in order to achieve a high-speed monochrome printing.
FIG. 1 is a block diagram showing a conventional configuration example of a density counting unit for totaling pixel values (density values) of an image formed on a photosensitive member. Further, FIG. 1 shows a case where laser diodes corresponding to respective C, M, Y, K1, K2 are used in order to achieve a high-speed monochromatic printing by way of example. Therefore, a density counting unit 2 includes density counters 2C, 2M, 2Y, 2K1, 2K2 for totaling pixel values (density values) of color components of C, M, Y, K1, K2, respectively.
As shown in FIG. 1, when two laser diodes are used for black, a rate of image formation is improved, while two density counters are required to be provided for black. Hence, problems occur such as an enlarged circuit scale, an increased power consumption and a remarkably risen manufacturing cost.